Photon Sphere Mechanics

Just outside the event horizon lies the photon sphere — a region where gravity is strong enough that light can orbit the black hole in unstable circular paths. For a non-spinning black hole, this occurs at 1.5 times the Schwarzschild radius, marking the closest a photon can circle without being pulled in.

Photons in the photon sphere are balanced on a knife's edge. Any slight perturbation sends them either spiraling into the black hole or escaping to infinity. This ring provides a direct measurement of the black hole's mass and spin.